Contrasting adsorption behavior of perfluorooctane sulfonic acid and its precursor perfluorooctane sulfonamide characterized by bench-scale-corroborated density-functional-theory analysis

The co-occurrence of perfluorooctane sulfonic acid (PFOS) and its precursors in soil and groundwater has been demonstrated for multiple sites. This study focuses on exploring and contrasting the adsorption behaviors of PFOS and its typical precursor perfluorooctane sulfonamide (PFOSA) via density functional theory (DFT) calculations. In a novel approach, the results of the DFT analyses are benchmarked and evaluated using measured adsorption data obtained under both static (surface-tension measurements) and dynamic (miscible-displacement experiments) conditions. DFT calculations reveal that the adsorption energy of PFOSA on the SiO₂ surface (−1.76 eV) is significantly more negative than that of PFOS (−0.85 eV), indicating a stronger adsorption of PFOSA on SiO₂ in neutral aqueous environments. This is attributed primarily to PFOSA's ability to form more hydrogen bonds with the SiO₂ surface through its sulfonyl and amide groups, whereas the dissociated form of PFOS restricts such hydrogen bond formation. The attribution of the adsorption-energy differences to headgroup-mediated interactions is supported by the results of the surface-tension measurements, which demonstrate that PFOSA and PFOS exhibit very similar magnitudes of hydrophobic interaction. The greater adsorption potential of PFOSA determined from DFT analysis is supported by the results of the miscible-displacement experiments, which show that PFOSA exhibits greater retardation compared to PFOS for transport in saturated quartz sand. Additional experiments conducted to examine the transport of mixed PFOS and PFOSA solutions reveal no apparent competitive adsorption, which is consistent with zeta-potential measurements of the PFAS-treated quartz sand surfaces. These findings deepen our understanding of PFOS and PFOSA adsorption behavior in the environment.